In certain geographical areas such as in portions of Africa, Europe and South and North America, large carnallite ore deposits consisting of primarily MgCl.sub.2.KCl.6H.sub.2 O with varying amounts of sodium chloride and other impurities are an ideal source for the recovery of magnesium chloride, which is then further processed into magnesium metal, and secondarily of potassium chloride for fertilizer manufacture. For some time, attempts have been made to render the carnallite processing more efficient. Carnallite is currently processed by solution in water with recovery of potassium chloride via a fractional crystallization process. Typically, an aqueous magnesium chloride rich brine is discarded during such an operation and the disposal of this can constitute a serious and costly environmental problem as well as a loss of a potentially profitable product.
More recently, new carnallite processes have been disclosed in which the carnallite is selectively separated through the solution of the magnesium chloride in methanol. Exemplary of such proceeses are East German Pat. No. 33,189 and U.S. Pat. No. 3,833,709. In accordance therewith, the methanol-magnesium chloride solution is filtered to separate the insoluble residue, primarily potassium chloride and sodium chloride. The methanol is then recovered, e.g., evaporated from the solution, combined with any methanol recovered from the filter cake and recirculated in a closed system.
In the above-referenced East German patent, the methanol recovery is performed in two steps. Methanol is evaporated from the solution in a first multi-stage vacuum evaporation and thereafter the remaining methanol is evaporated in a second, e.g. spray-drying step. The resulting methanol vapors are condensed in conventional equipment and recycled to the solubilizing step.
The vacuum evaporation, however, has serious drawbacks in that there exists a constant danger of an air-leak which, if present, can result in an explosive mixture. Thus, this process is not well suited for large scale commercial operations because of its safety hazard. Additionally, the drying is difficult to perform because of the economic need to recover essentially all of the methanol and to prevent the discharge of even trace quantities of methanol to the atmosphere because methanol is a toxic pollutant. Recirculating a suitable drying gas such as the exhaust gas of furnaces, boilers, etc. to the adsorptive methanol recovery process disclosed in the East German patent is too expensive to render it commercially feasible.
In the above-referenced U.S. patent, the shortcomings encountered with the process disclosed in the East German patent are largely eliminated. Generally speaking, the U.S. patent contemplates the evaporation of the methanol from the solution at an elevated temperature. The U.S. patent further discloses the addition of water to the solution, however, at a relatively late stage in the evaporation step. As a result, the solution becomes a relatively viscous mass which has a tendency to foul heat transfer surfaces which in turn can seriously interfere with the process. In addition, there may be local overheating of the mass which can lead to the decomposition of the magnesium chloride hydrate into highly corrosive hydrochloric acid and solid magnesium oxide which causes serious fouling of heat transfer surfaces and contaminates the subsequent magnesium chloride product unless removed.
As a refinement to the above-discussed prior art processes, and particularly the one disclosed in the referenced U.S. patent, it has also been contemplated to provide a twostep methanol evaporation in which the solution is concentrated in a first evaporation step and wherein the remainder is subsequently spray-dried in a closed recirculating system operated slighty above atmospheric pressure to avoid the safety hazards encountered in vacuum operations. However, the utility costs, and particularly the refrigeration and gas circulating power costs of operating such a system as well as its initial capital investment are presently too high to render its commercial use economically feasible.